Owners often come to the conclusion their vehicles are consuming excess fuel - but they are generally and overwhelmingly mistaken on this issue. What gives?

Mitsubishi Triton is one of the best 4X4 utes available - especially in the higher model grades

QUESTION

My name is Jake Turner and I'm trying to get in contact with John to obtain his help regarding an issue I'm having with Mitsubishi Motors and our Triton, in particular that we want to have our vehicle checked for excessive fuel consumption under warranty, but Mitsubishi are saying to us that we need to pay to have it looked at if they determine that the issue is not covered by warranty.

An overview of our issue:

In December, 2017 we bought a Triton from Adelaide and then drove it up to the Northern Territory. At the end of 2018 we drove back to South Australia from Nhulunbuy where I recall we received approximately 750km per tank of diesel (which is still higher than the advertised usage, however we expected that as it was a new car). At the beginning of November, 2019 we drove back up to the NT from Adelaide to Darwin, while driving we realised that we were consuming far more fuel than from other trips we have done in 2018.

I monitored the fuel usage from SA (110kmph) where we used around 12.3 - 12.9 litres of fuel per 100km. From the NT border (130kmph) we used around 13.1 to 13.7 litres of fuel per 100km. Since being in Darwin the usage shows 16.7 litres per 100km, however this is driving around in an urban environment, which I would expect to be higher than normal, but not this high. Once I realised we were consuming far more than we should, I checked the air filter, checked the oil, ensured adequate tyre air pressure and used diesel injector cleaner without success. We have used multiple different petrol stations along our journey up to the NT this month.

FYI - I note the advertised average Triton usage for our model is from 7.6 to 8.6 litres per 100km.

We have maintained excellent servicing, completing this every 10,000km instead of the recommended 15,000km. About one week prior to leaving SA we had it serviced (no more than 50km before trip).

In order to rectify this, I spoke with Mitsubishi in Darwin and advised them of the issue. They said they will look at it and, if deemed a warranty issue, they will rectify, however they then said that if it isn’t a warranty issue, then we would have to pay at minimum $175 and then an additional $175 per hour for labour.

Not happy with this response, I stressed that this is a two year old vehicle covered by warranty and that there is clearly an issue as I have written down the monitoring while we drove up from SA, to which I received the same response.

I then contacted Mitsubishi Head Office to speak with them, only to hear the exact same story.

I am frustrated that we clearly have an issue and that we need to have it looked at, but we had the vehicle serviced prior to leaving, noticed an issue that was different from previous drives on the same route, same conditions, same load etc. and even though it’s under warranty, they want to say that we may be liable for costs to just look at it.

I am in no way objecting to pay if it is something that needs to be fixed - that for some absurd reason is not covered under warranty, but to have the vehicle just looked at with the likelihood that I will need to pay is ridiculous.

I hope you can assist me.

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ANSWER

I get this all the time from people, and you’re probably not going to like my answer: Your Triton is probably fine. The problem is completely subjective, and stems from these issues:

Official fuel consumption figures

The official fuel tests are not ‘advertised fuel consumption’. They’re laboratory-standardised fuel tests conducted for homologation compliance. They’re done to a standard, in a controlled environment. Unfortunately, the standard is highly un-representative of real-world driving. Typically, actual consumption of all cars is 30+ per cent higher than advertised. This does not account for speed and load - aerodynamic drag increases with the square of speed, roughly, so there’s 40 per cent more drag at 130, compared with 110. And the official tests never go anywhere near 130.

If you have a roof rack, bullbar, lights, etc - drag will be be considerably worse. And presumably you drove the vehicle up there and back, loaded to some degree - so there’s that. More mass = higher fuel consumption.

Manufacturers’ hands are tied here. They’re not bullshitting you on consumption. They are legally compelled to use the lab test figures, and only the lab test figures in all official communications regarding their vehicles.

Errors in measured fuel consumption

People without applied scientific training - and I can tell you don’t have any by the way you’ve described this to me (not a criticism, but definitely a liability here) - are truly crap at measuring things like fuel consumption in a meaningful, experimentally controlled way. Sorry to be blunt. And, frankly, that’s how you have to do it - in a highly controlled way, if you want meaningful figures. So I would expect a considerable margin of error in your figures.

(Kays per tank is flat-out meaningless, for example. You cannot measure this accurately. It’s a pointless observation.) I’ve measured fuel consumption, a lot, as an engineer. It’s very hard to control - especially without using special equipment.

Urban fuel consumption for your Triton (lab test) is about 9L/100km. 16.7 is not unreasonable for a loaded vehicle in an urban environment. 12-14 is not unreasonable for high speed highway cruising in a hot environment. It’s a big vehicle pushing a lot of air aside.

Investigation

Your Triton is probably normal - you could check the DPF (if fitted) for undue back-pressure, and ask the dealership to do a forced regeneration of the DPF if the pressure is found to be excessive, and you could check the inlet tract for excessive crusty buildup flowing from PCV/EGR malfunction, but I think you will find the vehicle is operating normally. 13.7 L/100km is about 21 MPG in the old money, which does not seem unreasonable to me for such a large vehicle at high speed.

There is also a slim chance that the car could develop a leak in the inlet air plumbing, between the MAF sensor and the engine. If this happens, inlet air under pressure (from the turbocharger) would vent to atmosphere and the engine would continuously over-fuel under boost. (I’d suspect that if there was a heavy oily residue in the exhaust. This kind of malfunction can also compromise the DPF)

If those three things check out (inlet plumbing integrity, DPF and inlet build-up) then the car is probably operating normally.

People drive themselves insane with this - it’s quite common - but I urge you not to join this club. I strongly suspect the vehicle is operating normally - it’s just your expectations that need recalibrating.

Not what you want to hear, I know. Unfortunately I’m all about the facts.